This invention is based on the well-known theory of arc extinction by arc elongation. Since this theory has received little attention in the context of this invention, the relevant points of the theory of arc extinction by arc elongation will be briefly restated below.
Assume a linear fusible element of any metal, such as, e.g., silver or copper, is or is not embedded in an arc-quenching filler, such as quartz sand. Let the center of that fusible element be designated by the reference numeral 0, and points spaced equal distances to the left or the right from the center 0 be designated by the reference characters 1, 2, 3, 4, 5. This then yields the array of numerals indicated below:
5 4 3 2 1 0 1 2 3 4 5.
On low overloads the temperature distribution along this array is substantially parabolic, with a peak temperature at the center point 0, and negative gradients from 0 to both ends 5. Consequently melting and arc initiation occurs at the point 0 and a high initial arc voltage is developed at that point. That arc voltage is, however, far less than the arc voltage required to significantly decrease the current flow in the fusible element. As the fusible element burns back from 0 to 101, the arc voltage undergoes two changes. It is increased, on the one hand, by the elongation of the arc, and it is decreased, on the other hand, by the continued arcing time and the continued heat generation at the point 0 and the points immediately adjacent thereto. This may be expressed by the equation EQU V=V.sub.1-1 -V.sub.o.
Where V is the driving voltage, where V.sub.1-1 is the gain in voltage drop due to an elongation from 0 to 1-0-1 and where V.sub.o is the loss in voltage drop due to continued or prolonged heat generation and thermal ionization in the time required for the arc to reach the length 1-0-1. This continuous process of increase of arc voltage due to arc elongation and decrease of arc voltage due to prolonged heat generation and thermal ionization continues until the first and second term in the above equation are equal, and the driving voltage is consequently zero.
It is apparent from the above that the most effective means for extinguishing low current arcs must generally act on the center portion of the arc where the decrease of arc voltage with time is highest.
This invention relates to a particularly simple and cost-effective means for deionizing and cooling the center portion of an electric arc in a fuse, or other points thereof where arc-initiation occurs at load currents.